U.N. investigators say Islamic State militants' extreme use of violence against civilians and captured soldiers in northeast Syria amounts to war crimes and crimes against humanity.

A report from the U.N. Commission of Inquiry on Syria, titled “Rule of Terror: Living under ISIS in Syria,” describes in shocking detail the use of terror by Islamic State militants to subjugate Syrians living in its areas of control. The report is based on more than 300 interviews with victims and witnesses.

Since U.S.-led airstrikes began targeting Islamic State militants in Syria in late September, they have begun taking up positions in civilian houses and farms, leading to civilian casualties, the report said.

'Morality police'

It found that Islamic State was depriving 600,000 people in the north of deliveries of food and medical aid, and was enforcing its radical interpretation of Islamic law through "morality police."

The investigators said executions, amputations and public lashings occur regularly, and the display of mutilated bodies further terrorizes and traumatizes Syrians.

The report said Syrian women and girls are excluded from public life. Some have been stoned for having unapproved contact with the opposite sex. It said IS, also referred to as ISIS, dictates what women must wear, with whom they may socialize and where they may work.

Commission Chairman Paulo Pinheiro said children and minority communities also come in for particular abuse, as do journalists and activists trying to communicate the daily suffering of those living under IS control.

“The so-called ISIS has committed war crimes and crimes against humanity," the report said. "The abuses, violations and crimes committed by the so-called ISIS against Syrians have been deliberate and calculated. The commanders of ISIS have acted willfully, perpetrating these war crimes and crimes against humanity. They are individually criminally responsible for these crimes.”

Horrific abuse of children

The report said children have been the victims, perpetrators and witnesses of IS executions. It said boys under age 18 have been executed for alleged affiliation with other armed groups.

It said IS actively recruits children as soldiers and reportedly runs training camps for boys between the ages of 5 and 16 for combat roles.

Commission member Vitit Muntarbhorn said children are deliberately shown videos of mass executions to desensitize them to extreme violence. “A very worrying trend is indoctrination in education," he said. "So, whatever kids are there in the country under the control of the group, they are very much jeopardized and threatened by indoctrination and also being used as instruments to spread extremism.”

Though children may commit atrocities, Muntarbhorn said they are viewed as victims. It is the IS adults who are the perpetrators, he said.

The commission said the failure of the international community and the Syrian government to get involved in a political process to end the armed conflict in Syria has allowed extremism to grow. The report called on world powers to bring the IS commanders before the International Criminal Court for prosecution.

Iraqi forces

Also Friday, Iraqi officials said government forces had driven out Islamic State militants from the oil refinery town of Beiji, north of Baghdad.

The Islamic jihadists had captured Beiji during their blitz across northern and western Iraq earlier this year. The town is located on a highway leading to the key northern city of Mosul, which largely had fallen under the control of IS militants in June.

The news came a day after U.S. Defense Secretary Chuck Hagel told a congressional panel that attacks against the insurgents would intensify as Iraqi ground forces worked to become more effective.